More Loops Topics CounterControlled Definite Repetition EventControlled Indefinite

More Loops Topics • • • Counter-Controlled (Definite) Repetition Event-Controlled (Indefinite) Repetition for Loops do-while Loops Choosing an Appropriate Loop Break and Continue Statements Reading • Sections 5. 1 – 5. 11 CMSC 104, Version 8/06 L 13 More. Loops. ppt 1

Counter-Controlled Repetition (Definite Repetition) • If it is known in advance exactly how many times a loop will execute, it is known as a counter-controlled loop. int i = 1 ; while ( i <= 10 ) { printf(“i = %dn”, i) ; i=i+1; } CMSC 104, Version 8/06 L 13 More. Loops. ppt 2

Counter-Controlled Repetition (con’t) • Is the following loop a counter-controlled loop? while ( x != y ) { printf(“x = %d”, x) ; x=x+2; } CMSC 104, Version 8/06 L 13 More. Loops. ppt 3

Event-Controlled Repetition (Indefinite Repetition) • If it is NOT known in advance exactly how many times a loop will execute, it is known as an event-controlled loop. sum = 0 ; printf(“Enter an integer value: “) ; scanf(“%d”, &value) ; while ( value != -1) { sum = sum + value ; printf(“Enter another value: “) ; scanf(“%d”, &value) ; } CMSC 104, Version 8/06 L 13 More. Loops. ppt 4

Event-Controlled Repetition (con’t) • An event-controlled loop will terminate when some event occurs. • The event may be the occurrence of a sentinel value, as in the previous example. • There are other types of events that may occur, such as reaching the end of a data file. CMSC 104, Version 8/06 L 13 More. Loops. ppt 5

The 3 Parts of a Loop #include <stdio. h> int main () { int i = 1 ; /* count from 1 to 100 */ while ( i < 101 ) { printf (“%d “, i) ; i=i+1; } return 0 ; initialization of loop control variable test of loop termination condition modification of loop control variable } CMSC 104, Version 8/06 L 13 More. Loops. ppt 6

The for Loop Repetition Structure • The for loop handles details of the counter-controlled loop “automatically”. • The initialization of the loop control variable, the termination condition test, and control variable modification are handled in the for loop structure. for ( i = 1; i < 101; i = i + 1) { initialization modification } test CMSC 104, Version 8/06 L 13 More. Loops. ppt 7

When Does a for Loop Initialize, Test and Modify? • Just as with a while loop, a for loop o initializes the loop control variable before beginning the first loop iteration, o modifies the loop control variable at the very end of each iteration of the loop, and o performs the loop termination test before each iteration of the loop. • The for loop is easier to write and read for counter-controlled loops. CMSC 104, Version 8/06 L 13 More. Loops. ppt 8

A for Loop That Counts From 0 to 9 for ( i = 0; i < 10; i = i + 1 ) { printf (“%dn”, i) ; } CMSC 104, Version 8/06 L 13 More. Loops. ppt 9

We Can Count Backwards, Too for ( i = 9; i >= 0; i = i - 1 ) { printf (“%dn”, i) ; } CMSC 104, Version 8/06 L 13 More. Loops. ppt 10

We Can Count By 2’s. . . or 7’s … or Whatever for ( i = 0; i < 10; i = i + 2 ) { printf (“%dn”, i) ; } CMSC 104, Version 8/06 L 13 More. Loops. ppt 11

The do-while Repetition Structure do { statement(s) } while ( condition ) ; • The body of a do-while is ALWAYS executed at least once. Is this true of a while loop? What about a for loop? CMSC 104, Version 8/06 L 13 More. Loops. ppt 12

Example do { printf (“Enter a positive number: “) ; scanf (“%d”, &num) ; if ( num <= 0 ) { printf (“n. That is not positive. Try againn”) ; } } while ( num <= 0 ) ; CMSC 104, Version 8/06 L 13 More. Loops. ppt 13

An Equivalent while Loop printf (“Enter a positive number: “) ; scanf (“%d”, &num) ; while ( num <= 0 ) { printf (“n. That is not positive. Try againn”) ; printf (“Enter a positive number: “) ; scanf (“%d”, &num) ; } • Notice that using a while loop in this case requires a priming read. CMSC 104, Version 8/06 L 13 More. Loops. ppt 14

An Equivalent for Loop printf (“Enter a positive number: “) ; scanf (“%d”, &num) ; for ( ; num <= 0; ) { printf (“n. That is not positive. Try againn”) ; printf (“Enter a positive number: “) ; scanf (“%d”, &num) ; } • A for loop is a very awkward choice here because the loop is event-controlled. CMSC 104, Version 8/06 L 13 More. Loops. ppt 15

So, Which Type of Loop Should I Use? • Use a for loop for counter-controlled repetition. • Use a while or do-while loop for eventcontrolled repetition. o o Use a do-while loop when the loop must execute at least one time. Use a while loop when it is possible that the loop may never execute. CMSC 104, Version 8/06 L 13 More. Loops. ppt 16

Nested Loops • Loops may be nested (embedded) inside of each other. • Actually, any control structure (sequence, selection, or repetition) may be nested inside of any other control structure. • It is common to see nested for loops. CMSC 104, Version 8/06 L 13 More. Loops. ppt 17

Nested for Loops for ( i = 1; i < 5; i = i + 1 ) { for ( j = 1; j < 3; j = j + 1 ) { if ( j % 2 == 0 ) { printf (“O”) ; } else { printf (“X”) ; } } printf (“n”) ; } CMSC 104, Version 8/06 L 13 More. Loops. ppt How many times is the “if” statement executed? What is the output ? 18

The break Statement • The break statement can be used in while, do-while, and for loops to cause premature exit of the loop. • THIS IS NOT A RECOMMENDED CODING TECHNIQUE. CMSC 104, Version 8/06 L 13 More. Loops. ppt 19

Example break in a for Loop #include <stdio. h> int main ( void ) { OUTPUT: int i ; for ( i = 1; i < 10; i = i + 1 ) 1234 { if (i == 5) Broke out of loop at i = 5. { break ; } printf (“%d “, i) ; } printf (“n. Broke out of loop at i = %d. n”, i) ; return 0 ; } CMSC 104, Version 8/06 L 13 More. Loops. ppt 20

The continue Statement • The continue statement can be used in while, do-while, and for loops. • It causes the remaining statements in the body of the loop to be skipped for the current iteration of the loop. • THIS IS NOT A RECOMMENDED CODING TECHNIQUE. CMSC 104, Version 8/06 L 13 More. Loops. ppt 21

Example continue in a for Loop #include <stdio. h> int main ( ) { int i ; for ( i = 1; i < 10; i = i + 1 ) { if (i == 5) { continue ; } printf (“%d ”, i) ; } printf (“n. Done. n”) ; return 0 ; } CMSC 104, Version 8/06 L 13 More. Loops. ppt OUTPUT: 12346789 Done. 22

Off By One Error #define TEN 10 int i; for ( i = 1; i < TEN; i = i + 1 ) { printf( “%dn”, i ); } /* Does not count from one to ten, only one to nine. */ CMSC 104, Version 8/06 L 13 More. Loops. ppt 23

Another Popular Mistake i = 1; while ( i < 10 ) printf( “i is %dn”, i ); i = i + 1; /* end while */ • i never changes, because only the printf is in the while loop! */ CMSC 104, Version 8/06 L 13 More. Loops. ppt 24